Wavelength-division multiplexer are employed in optical fiber systems in which light waves of a particular spectrum or channel may be either multiplexed, i.e. added to, or demultiplexed, i.e. extracted from, a given transmission traveling in a core mode of a fiber optic cable. A channel extracted from a fiber optic cable is to be transferred to a second fiber optic cable while a channel being added to the first fiber optic cable are transferred from the second fiber optic cable. Various devices have been developed for transferring wavelength channels between two optical fiber cables.
One device is taught by Bilodeau et al., in "An all-fiber dense wavelength division multiplexer/demultiplexer using photoimprinted Bragg gratings" IEEE Photonics Technology Letters, Vol. 7, No. 4, pp. 388-390, which provides an all-fiber Mach-Zehnder interferometer with identical Bragg gratings in each of a pair of interferometer arms formed between two identical fused coupler regions. However, due to the interferometric nature of the device, the location of each component, i.e. the couplers, the fiber Bragg gratings, and the interfering arms needs to be controlled to a high degree of accuracy, thereby impeding its high-volume production.
A grating-frustrated coupler is disclosed by Arhambault et al. in Optics Letters Vol. 19 No. 3 Feb. 1, 1994, pp. 180-182, in which two single mode fibers form a 2.times.2 directional coupler. The two fiber cores are identical, except that only one of the cores contains a photorefractive Bragg grating coextensive with the coupling region. The grating frustrates the transfer of optical power from one fiber to the other within a narrow wavelength range. This approach also uses polished couplers which are difficult and expensive to fabricate. Additionally, the environmental stability of polished couplers tends to be lower than that of other approaches such as the fused taper method.
A grating-assisted coupler as disclosed by Dong et al. in IEEE Photonics Technology Letters, Vol. 8 No. 12, December 1996 includes a grating written in one fiber core over the coupling region of a polished 1000% coupler. The dropped channel sees a mismatched coupler due to the existence of the fiber Bragg grating and is therefore not coupled. Again, the environmental stability of polished couplers tends to be lower than that of other approaches such as the fused taper method.
U.S. Pat. No. 5,703,976 to Cullen discloses a wavelength selective 2.times.2 single mode fiber coupler having a weakly coupled coupling region in which the cladding modes of the two fibers are optically coupled while the core modes remain uncoupled. A matched pair of core-cladding mode converters are located on each fiber to opposite sides of the cladding region. This technique, however uses optical notch filters or bending to achieve conversion between the core and cladding modes. The forward propagating core mode is coupled to a forward propagating cladding mode and vice versa. Coupling from one forward propagating mode to another forward propagating mode requires a mode converter with a large period. Hence, in order to provide efficient coupling, these gratings must be relatively long. Furthermore, the notch filter and bending approaches are relatively difficult to control, particularly for narrow band devices.
It is therefore desirable to provide an add/drop filter for an optical fiber which may be more easily and reliably fabricated and without the need for both precisely positioning the components with respect to each other and for phase matching the components.